This really depends on the reason you want to learn. For back-end development I would orient towards Golang. Its approach to concurrency makes efficiently util

This really depends on the reason you want to learn. For back-end development I would orient towards Golang. Its approach to concurrency makes efficiently utilizing multiple cores a breeze; something that is not quite so simple in Node/PHP. As well, it's common that you'll find yourself able to get more out of individual machines - this will allow you to get more money out of said machines and, as a second-order effect, can delay the onset of scaling concerns. As an aside, as a primarily JavaScript developer, I do have to admit Go has a very impressive stdlib. For taming your tools but not changing ecosystems I would recommend looking into TypeScript. It just being a superset of JavaScript means you can do incremental adoption while still enjoying all of the JS knowledge you've built to date. For learning new paradigms I would either recommend Haskell for a "strict functional" path or Rust for an "immutable/non-racy" approach. Both languages take the concepts to a pretty extreme level and may feel overwhelming at first, but they can be brought back to your existing back-end development skills to help enhance the maintainability of your code. For something between concurrency and functional programming I would recommend Erlang/Elixir - though I do so only from recommendations and not from any meaningful experience with either. Might be up your alley and definitely worth at least considering for your "one language".